1. Field of the Invention
The invention relates to sensors for sensing pressure in a fluid flow line. More particularly, the invention relates to a sensor for sensing differential pressure across an orifice plate in a fluid flow line and for sensing gauge pressure upstream or downstream of the orifice plate.
2. Brief Description of the Prior Art
There are many industrial applications which require the measurement of pressures and temperatures in fluid flow lines.
The oil and gas and chemical processing industries, for example, utilize fluid flow line sensors for a variety of reasons. One common utilization of fluid flow line sensors is to determine flow rate, and thus the volume or mass of fluid passing through a flow line.
Often these measurements are used to compute the cost of a fluid such as hydrocarbon gas which is transferred by pipe from a supplier to a purchaser. It is therefore important that the measurements be as accurate as possible.
State-of-the-art fluid flow computers utilize pressure and temperature measurements to determine flow rate. For example, prior art FIG. 1 illustrates pressure/temperature transmitter 2 which is mechanically coupled to a flow line 4 on either side of a orifice plate 6.
A temperature sensor 8 is provided downstream of the orifice plate 6 and is electrically coupled to the transmitter 2 by a cable 10 through an explosion proof boss 12. The transmitter 2 transmits differential pressure, absolute pressure, and process temperature from an electronics package 14 through a cable 20 to another device (not shown). Pressure sensors (not shown) are housed in housing 16.
The orifice plate 6 causes a drop in pressure from the upstream pressure P1 to the downstream pressure P2 so that a differential pressure may be measured. Different state-of-the-art pressure sensors sense pressure in different ways.
Differential pressure is preferably sensed using a piezoresistive silicon membrane where one side of the membrane is exposed to upstream fluid pressure and the other side of the membrane is exposed to downstream fluid pressure.
Prior art FIG. 2 shows an example of a "meter body" 22 which is used for sensing differential pressure. The meter body 22 generally includes a stainless steel diaphragm assembly 24, a sensor header 26, and a sensor cover 28. The diaphragm assembly 24 includes high pressure diaphragm 30, a low pressure diaphragm 32, and a differential pressure diaphragm 34. The high pressure diaphragm 30 is exposed to upstream high pressure P1 and the low pressure diaphragm 32 is exposed to downstream low pressure P2.
A plurality of contiguous cavities 36 between the high pressure diaphragm 30 and the differential pressure diaphragm 34 are filled with an appropriate fluid such as silicon oil via a fill port 38. Similarly, a plurality of contiguous cavities 40 between the low pressure diaphragm 32 and the differential pressure diaphragm 34 are filled with silicon oil via a fill port 42. High pressure silicon oil is conducted from cavities 36 to a location on one end of the diaphragm assembly 24 via a pressure port 44. Low pressure silicon oil is conducted from cavities 40 to another location at the same end of the diaphragm assembly via a pressure port 46. The sensor header 26 and sensor cover 28 are attached to the end of the diaphragm assembly 24 where the ports 44 and 46 terminate.
The sensor header 26 houses a glass tube 48 with a piezoresistive silicon membrane 50 attached to one end. The other end of the tube 48 is coupled to a radial bore 52 which extends to the side surface of the header 26. The sensor cover 26 is dimensioned to provide an annular space 54 between the sensor header 26 and sensor cover 28. The sensor header 26 and sensor cover 28 are coupled to the diaphragm assembly 24 such that the high pressure port 44 is in fluid communication with one side 50a of the membrane 50 and the low pressure port is in fluid communication with the other side 50b of the membrane 50.
It will be appreciated from FIG. 2 that the side 50b is brought into fluid communication with the port 46 via the tube 48, the bore 52, and the annular space 54.
The piezoresistive silicon membrane 50 is essentially a strain gauge. As illustrated in prior art FIGS. 3 and 3A, four electrical resistors 56a-56d are embedded in the silicon membrane 50 to form a Wheatstone bridge.
Those skilled in the art will appreciate that the change in voltage Delta V across two nodes of the bridge shown in FIG. 3A is proportional to the differential pressure dP=P1-P2. As illustrated schematically in FIGS. 3 and 3A, the membrane 50 is rectangular having an overall thickness "T" and a central circular portion of reduced thickness "t". The ratio of T:t is approximately 50:1 and t is approximately 1 mil. The resistors 56a-56d are preferably arranged at the interface between thickness T and thickness t and according to the lattice structure of the silicon in order to provide the most accurate measurements.
According to some prior art methods, the differential pressure together with temperature is useful in calculating flow rate. A prior art pressure sensor utilizing the principles illustrated in FIGS. 2, 3, and 3A is the Honeywell ST3000 Smart Transmitter.
According to other methods, it is necessary or desirable to measure the static pressure of the fluid, either upstream or downstream. A prior art sensor which is capable of measuring both differential pressure and static pressure is the Honeywell SMV3000 Smart Multivariable Transmitter which is illustrated schematically in prior art FIGS. 4 and 4A.
As shown in FIGS. 4 and 4A, a piezoresistive silicon membrane 50' is mounted on the end of a glass tube 48'. The membrane 50' differs from the membrane 50 described above in that it has a second area of reduced thickness t' which overlies a portion of the wall of the tube 48' and which defines a space 58 containing a vacuum. A second strain gauge comprising resistors 60a-60d is arranged around this portion t'. One side 50'c of the membrane at are t' is exposed to upstream high pressure P1 and the other side 50'd is exposed to vacuum.
Those skilled in the art will appreciate that the voltage differential measured by the strain gauge 60a-60d will be proportional to the absolute pressure Plabs. Given the pressure differential dP and the absolute upstream pressure Plabs, the absolute downstream pressure P2abs is known by Plabs-dP.
With these and some other measurements, mass flow rate and other variables can be calculated. U.S. Pat. No. 5,606,513, the complete disclosure of which is hereby incorporated herein by reference, discloses several methods for utilizing the pressure and temperature measurements.
The Honeywell SMV-3000 is compact, efficient, relatively inexpensive, and is capable of measuring differential and absolute pressure in a single assembly. However, it is sometimes desirable to measure static pressure as gauge pressure rather than absolute pressure. One reason why gauge pressure is more desirable than absolute pressure is that it is easier to calibrate sensors using an atmospheric pressure rather than a vacuum as the reference pressure.
The previously incorporated '513 patent discloses an apparatus for measuring differential pressure dP, upstream absolute pressure P1abs, and upstream gauge pressure P1gauge. However, the apparatus requires several assemblies, is complex, mixes capacitive and piezoresistive measurements, and is relatively expensive.